Much work has been done to explore hydrophobicity and hydrophilicity in surfaces. Hydrophobic surfaces repel aqueous liquids, and hydrophilic surfaces attract aqueous liquids. A high degree of surface roughness generally results in a substantial increase in the hydrophobicity of a solid substrate, which accordingly exhibits extremely high water repellency and very low aqueous flow resistance. Such behavior makes superhydrophobic surfaces very attractive for a wide range of applications ranging from microfluidics and lab-on-a-chip devices, to drag reduction coatings and self-cleaning coatings. There also has been increasing research interest directed to enabling reversible tuning of surface wettability in order to dynamically switch the surface properties, including droplet mobility, contact angle, and the effective area of the solid-liquid interface, all from the same substrate. Such a level of dynamic control would potentially allow fabrication of smart surfaces capable of manipulating liquids at the micro- and macro-scales.
For example, surfaces have been prepared having an initially hydrophobic state, which can be induced by an external stimulus to become hydrophilic. Hence, an aqueous liquid in contact with the surface in its initial form cannot wet the surface, but can wet the surface after conversion to the hydrophilic state. However, once such surfaces are wet by an aqueous liquid, removal of the liquid and restoration of the surface to a hydrophobic state typically is quite difficult if not virtually impossible. Hence, such surfaces often can only be activated once to switch from the hydrophobic to a hydrophilic state. Even for such one time switching, these surfaces can be useful. For example, movement of an aqueous liquid over such a surface is impeded by a hydrophobic state, and facilitated by a hydrophilic state. However, the one way and one use limitations posed by such surfaces is a restraint on their utility.
In one prior approach to achieve reversibility of a hydrophilic surface to a hydrophobic state, solvents were selectively applied to the surface to change the character of hydrophilic and hydrophobic fibers both bonded to the surface. See, for example, Minko, S. et al., “Two-Level Structured Self-Adaptive Surfaces with Reversibly Tunable Properties”, J. Am. Chem. Soc., Vol. 125, pp. 3896-3900 (2003). However, conversions of the surface between hydrophilic and hydrophobic states in this system were slow, messy, and generally allowed no spatial control of the state changes across the surface.
There is accordingly a continuing need for surfaces that are reversible between hydrophobic and hydrophilic states.